Astronomers from the Sloan Digital Sky Survey Make the Most Precise Measurement Yet of the Expanding Universe

Astronomers from the Sloan Digital Sky Survey have used 140,000 distant quasars to measure the expansion rate of the Universe when it was only one-quarter of
its present age. This is the best measurement yet of the expansion rate at any epoch in the last 13 billion years.

The Baryon Oscillation Spectroscopic Survey (BOSS), the largest component of the third Sloan Digital Sky Survey (SDSS-III), pioneered the technique of
measuring the structure of the young Universe by using quasars to map the distribution of intergalactic hydrogen gas. Today, new BOSS observations of this
structure were presented at the April 2014 meeting of the American Physical Society in Savannah, GA.

An artist's conception of how BOSS uses quasars to measure the distant universe.
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An artist's conception of how BOSS uses quasars to measure the distant universe. Light from distant quasars is partly absorbed by intervening gas,
which is imprinted with a subtle ring-like pattern of known physical scale. Astronomers have now measured this scale with an accuracy of two percent,
precisely measuring how fast the universe was expanding when it was just 3 billion years old.
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These latest results combine two different methods of using quasars and intergalactic gas to measure the rate of
expansion of the Universe. The first analysis, by Andreu Font-Ribera (Lawrence Berkeley National Laboratory) and
collaborators, compares the distribution of quasars to the distribution of hydrogen gas to measure distances in the Universe.
A second analysis team led by Timothée Delubac (École Polytechnique Fédérale de Lausanne, Switzerland)
focused on the patterns in the hydrogen gas itself to measure the distribution of mass in the young Universe. Together the
two BOSS analyses establish that 10.8 billion years ago, the Universe was expanding by one percent every 44 million years.

"If we look back to the Universe when galaxies were three times closer together than they are today, we'd see that a pair of
galaxies separated by a million light-years would be drifting apart at a speed of 68 kilometers per second as the Universe
expands," says Font-Ribera.

Delubac explains that "we have measured the expansion rate in the young Universe with an unprecedented precision of 2 percent."
Measuring the expansion rate of the Universe over its entire history is key in determining the nature of the dark energy that is
responsible for causing this expansion rate to increase during the past six billion years. "By probing the Universe when it was only
a quarter of its present age, BOSS has placed a key anchor to compare to more recent expansion measurements as dark energy has taken
hold," says Delubac.

BOSS determines the expansion rate at a given time in the Universe by measuring the size of baryon acoustic oscillations (BAO),
a signature imprinted in the way matter is distributed, resulting from sound waves in the early Universe. This imprint is visible
in the distribution of galaxies, quasars, and intergalactic hydrogen throughout the cosmos.

An illustration of how astronomers used quasar light to trace the expansion of the universe.
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An illustration of how astronomers used quasar light to trace the expansion of the universe. The expansion is shown
by the increasing circles from right to left. From the Big Bang, the expansion occurs rapidly, then slows down, then speeds up again
as dark energy pushes apart walls and filaments of galaxies at different distances (purple). As light travels to us from very distant
quasars (white dots on the right), it passes through the expanding universe, carrying with it the story of its journey through this
cosmic web. Astronomers have measured the expansion of the universe by tracing how quasar light has passed through these
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"Three years ago, BOSS used 14,000 quasars to demonstrate we could make the biggest 3-D maps of the Universe," says David
Schlegel (Lawrence Berkeley National Laboratory), principal investigator of BOSS. "Two years ago, with 48,000 quasars, we first
detected baryon acoustic oscillations in these maps. Now, with more than 140,000 quasars, we've made extremely precise measures of BAO."

As the light from a distant quasar passes through intervening hydrogen gas distributed throughout the Universe, patches of greater
density absorb more light. Each absorbing patch absorbs light from the spectrum of the quasar at a characteristic wavelength of neutral
hydrogen. As the Universe expands, the quasar spectrum is stretched out, and each subsequent patch leaves its absorption mark at a different
relative wavelength. The quasar spectrum is finally observed on Earth by BOSS, and it contains the signatures of all the patches encountered
by the quasar light. Astronomers then measure from the quasar spectrum how much the Universe has expanded since the light passed through
each patch of hydrogen.

With enough good quasar spectra, close enough together, the position of the gas clouds can be mapped in three dimensions. BOSS determines
the expansion rate by using these maps to measure the size of the BAO pattern at different epochs of cosmic time. These new measurements
provide key data for astronomers seeking the nature of the dark energy postulated to be driving the increase in the expansion rate of the
Universe.

David Schlegel remarks that when BOSS was first getting underway, precision measurements using quasars and the Lyman-alpha forest had
been suggested, but "some of us were afraid it wouldn't work. We were wrong. Our precision measurements are even better than we optimistically
hoped for."

About SDSS-III

Funding for SDSS-III has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the
National Science Foundation, and the U.S. Department of Energy's Office of Science. This research used resources of
the National Energy Research Scientific Computing Center (NERSC), which is supported by the Office of Science.
Visit SDSS-III at http://www.sdss3.org.

SDSS-III is managed by the Astrophysical Research Consortium for the Participating Institutions of the SDSS-III
Collaboration including the University of Arizona, the Brazilian Participation Group, Brookhaven National Laboratory,
University of Cambridge, Carnegie Mellon University, University of Florida, the French Participation Group, the German
Participation Group, Harvard University, the Instituto de Astrofisica de Canarias, the Michigan State/Notre Dame/JINA
Participation Group, Johns Hopkins University, Lawrence Berkeley National Laboratory, Max Planck Institute for
Astrophysics, Max Planck Institute for Extraterrestrial Physics, New Mexico State University, New York University,
Ohio State University, Pennsylvania State University, University of Portsmouth, Princeton University, the Spanish
Participation Group, University of Tokyo, University of Utah, Vanderbilt University, University of Virginia,
University of Washington, and Yale University.